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Annual Water Yield model Z parameter


I have a question about the calculation of the Z parameter for the Annual Water Yield model.

This formula is given in the InVEST manual: Z=((ω−1.25)P )/ AWC (where P and AWC should be average values of Precipitation and Available Water Capacity, respectively, in the study area). 

I found a formula for ω in the following paper (see attached file): Xu, X., Liu, W., Scanlon, B. R., Zhang, L., & Pan, M. (2013). Local and global factors controlling water‐energy balances within the Budyko framework. Geophysical Research Letters, 40(23), 6123-6129.
This formula however uses the basin center, drainage area in the formula.
My study area includes parts of several basins (but never one complete basin). I only have information for the study area so running the model for each separate basin is not possible.

How should I handle this problem to be able to run the model for my study area?

Thank you in advance?

Looking forward to your answer!

Kind regards


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